Most rubbery materials are electric insulators. A number of conductive rubbers are available as mixtures of rubbery material and conductive agents. For example, rubbers having carbon black blended therein to provide an electric resistivity of from 10.sup.5 to 10 .OMEGA..cm are conductive rubbers which are used in various applications.
Silicone rubber is also widely used as electrically insulating rubber because of its high-temperature resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents like other rubbery materials. It is also known to add blowing agents thereto whereupon the composition is expanded and cured into conductive silicone rubber foams.
However, only a limited vulcanization system is applicable to conductive silicone rubber compositions having blended therein acetylene black and other carbon black as a conductive agent when they are continuously molded as by extrusion molding and vulcanized into a length of article such as a seal, gasket and roll. More particularly, typical organic peroxide vulcanization fails to yield satisfactory molded products. If acyl series peroxides commonly used in atmospheric hot air vulcanization (HAV) of conventional silicone rubber compositions, for example, benzoyl peroxide and 2,4-dichlorobenzoyl peroxide are used in carbon black-containing systems, the carbon black acts to restrain vulcanization. If alkyl series peroxides, for example, dicumyl peroxide and di-tert-butyl peroxide are used in carbon black-containing systems, compression molding is permissible, but extrusion atmospheric hot air vulcanization fails to yield satisfactory products because the surface is vulcanized short due to the influence of oxygen in air.
For extrusion atmospheric hot air vulcanization of carbon black-containing silicone rubber compositions, an addition vulcanization approach was conventionally used in order to overcome the above-mentioned difficulty. This approach is to add a platinum series addition reaction catalyst to a mixture of an organopolysiloxane having an alkenyl group and an organohydrogenpolysiloxane having a silicon-attached hydrogen atom capable of addition reaction with the alkenyl group, thus inducing addition reaction curing.
This addition reaction curing approach, however, suffers from the problems of a limited shelf life, a short pot life, and poisoning by catalyst poisons such as amines, sulfur and tin. When it is desired to obtain a satisfactory expanded product or foam, vulcanization must be carried out to a certain extent (until scorching) through addition vulcanization in order to confine within the system a blowing gas resulting from decomposition of a blowing agent. It is then necessary to look for an optimum control agent for controlling addition reaction. Even if an appropriate control agent is used, the permissible pot life to produce a satisfactory foam is limited to a very narrow time duration. It is then substantially impossible for an addition reaction system to continuously produce a conductive silicone rubber foam through atmospheric hot air vulcanization. There is a desire to have an expandable conductive silicone rubber composition which has a long pot life which is acceptable by the worker engaged in sponge molding and is rapidly vulcanizable into a silicone rubber foam having improved physical properties.